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This chapter introduces leaves by comparing them with solar panels and by discussing their general functions, morphology, and dimensions. This is followed by descriptive information on basic leaf types and specific forms and arrangements. The chapter next discusses the internal structure of leaves, including epidermis and cuticle, stomata, glands, mesophyll, and veins.

Specialized leaves, including tendrils, spines, flower-pot leaves, window leaves, reproductive leaves, floral leaves, and insectivorous leaves, are then examined. The chapter concludes with an explanation of autumnal color changes and abscission and some observations on the human and ecological relevance of leaves.

Some Learning Goals

Learn the external forms and parts of leaves. Know the functions of a typical leaf and the specific tissues and cells that contribute to those functions.

Understand the differences among pinnate, palmate, and dichotomous venation and also the differences between simple and compound leaves.

Contrast tendrils, spines, storage leaves, flower-pot leaves, window leaves, reproductive leaves, floral leaves, and different types of insect-trapping leaves.

Explain why deciduous leaves turn various colors in the fall and how such leaves are shed.

Know at least 15 uses of leaves by humans.

he earliest records of glass being used by humans

Tdate back to about 2600 B.C., when the ancient Egyptians and Babylonians made beads from the material. The use of glass panes for windows, however, did not begin until the Roman Imperial period a little over 2,000 years ago. Since then, the use of glass windows for admitting light to buildings of all sizes and shapes has become almost universal.

A comparatively recent use of glass involves solar energy as an alternative to nonrenewable sources of energy such as fossil fuels. The construction industry, particularly in the southwestern United States, is building new houses with flat panels and windows inclined at angles that maximize the amount of energy captured from the sun's rays. Some buildings have mechanical devices that slowly move the solar panels so that they will follow the sun in its daily course across the sky. The use of such means of capturing solar energy is now spreading to other countries and could soon become commonplace.

Plants had a highly efficient form of solar panel that captured the sun's energy many aeons before modern civilization began to realize that fossil-fuel supplies eventually would be exhausted. These remarkably constructed solar panels are the plant organs known to us as leaves.

All leaves originate as primordia in the buds, regardless of their ultimate size or form. In early spring, a leaf pri-mordium may consist of fewer than 200 cells, but in response to changes in temperature, day length, and availability of water, hormones are produced that stimulate these cells to begin dividing. Within a few days or weeks, the original 200 cells have multiplied, differentiated, and expanded into a structure consisting of millions of cells. In some plants (e.g., Eucalyptus), the first leaves produced (juvenile leaves) may appear quite different in form from those produced later. The juvenile form may be promoted by a class of hormones known as gibberellins (discussed in Chapter 11).

At maturity, most leaves have a stalk, called the petiole, and a flattened blade, or lamina, which has a network of veins (vascular bundles). A pair of leaflike, scalelike, or thornlike appendages, called stipules, are sometimes present at the base of the petiole. Occasionally, leaves may lack petioles; when they do, they are said to be sessile. Leaves of deciduous trees normally live through only one growing season, and even those of evergreen trees rarely function for more than 2 to 7 years.

Leaves of flowering plants are associated with leaf gaps (as illustrated in Fig. 6.3), and all have an axillary bud at the base. Leaves may be simple or compound. A simple leaf has a single blade, while the blade of a compound leaf is divided in various ways into leaflets (see Fig. 7.4). Regardless of the number of leaflets, a compound leaf still has a single axillary bud at its base, with the leaflets having no such buds. Pinnately compound leaves have the leaflets in pairs along an extension of the petiole called a rachis, while palmately compound leaves have all the leaflets attached at the same point at the end of the petiole. Sometimes, the leaflets of a pinnately compound leaf may be subdivided into still smaller leaflets, forming a bipinnately compound leaf (Fig. 7.1).

The flattened surfaces of leaves, which are completely covered with a transparent protective layer of cells, the epidermis, admit light to all parts of the interior. Many leaves twist daily on their petioles so that their upper surfaces are inclined at right angles to the sun's rays throughout daylight hours (Fig. 7.2).

Green leaves capture the light energy available to them by means of the most important process for life on earth, at least life as we know it today. This process, called photosynthesis (discussed in Chapter 10), involves the trapping and ultimate storing of energy in sugar molecules that are constructed from ordinary water and from carbon dioxide present in the atmosphere. All the energy needs of living organisms ultimately depend on photosynthesis, from the first day of their existence to the last.

Figure 7.2 Algerian ivy on a tree trunk. Note how each leaf blade is oriented to receives the maximum amount of light.

The lower surfaces of leaves (and in some plants, the upper surfaces as well) are dotted with tiny pores (stomata), which not only allow entry for the carbon dioxide gas needed for photosynthesis, but also play a role in the diffusion out of the leaf of oxygen produced during photosynthesis. Water vapor evaporating from the moist interior cell surfaces can also escape via the stomata. The evaporation of water can bring about some cooling of the leaf, but excessive water loss can result in damage to the plant. The stom-atal apparatus, which consists of a pore bordered by a pair of sausage-shaped guard cells, controls the water loss when the guard cells inflate or deflate, opening or closing the pore.

Leaves also perform other functions. For example, all living cells respire (respiration is also discussed in Chapter 10), and in the process of this and other metabolic activities, waste products are produced. These wastes accumulate in the leaves and are disposed of when the leaves are shed, mostly in the fall. Before dropping from the plant, the leaves are sealed off at the bases of their petioles (see the discussion of leaf colors and abscission later in this chapter). The following season, the discarded leaves are replaced with new ones.

Leaves play a major role in the movement of water absorbed by roots and transported throughout the plant. Most of the water reaching the leaves evaporates in vapor form into the atmosphere by a process known as transpiration (discussed in Chapter 9). In some plants, there are special openings called hydathodes at the tips of leaf veins. Root pressure (see page 161) forces liquid water out of hydathodes, usually at night when transpiration is not occurring. The loss of water through hydathodes is called guttation. The expelled water may contain ions secreted by root cells. Other functions of leaves are discussed throughout this chapter.

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